Generally, in the semiconductor, electronic circuit and electronic instrument, various electronic components are fixed on a substrate with fusion of solder, and the electronic continuity is secured. However, the conventional solder is an alloy of Sn and Pb, and the applications of Pb are being prohibited as a recent environment safeguard measure, so that Pb-free solder is being developed instead of said conventional solder. The melting point of eutectic solder of Sn and Pb is 183° C. and the melting point of Sn.Ag.Cu solder being conventional substitute solder is 217° C. In a case that the soldering is done to a resin substrate, resinous heat resistance is low, and since the melting point of conventional substitute solder is too high, the resin substrate is often damaged, so that solder for low-temperature was requested. Considering from the melting point of said conventional substitute solder, the substitute solder which can connect in a region equal to or less than 200° C. is desired.
As the characteristics of substitute solder, it is natural that Pb is not contained and the metalization temperature is low, and besides it is required that safety is high, there is no corrosion property, and the electrical conductivity and the heat conductivity are good. This situation is requested. Silver attracts attention as a material meeting this expectation. Besides, in order to decrease the melting point, the composite silver nanoparticle being ultrafine particle has been developed.
The patent bulletin of JP3205793 (Japanese Patent Laid-Open No. H10-183207) is published as patent document 1. A silver organic compound (a silver organic complex in particular) is chosen as a starting material. Under the inert gas ambient atmosphere that intercepted air, said silver organic compound is heated in a temperature range higher than the decomposition start temperature and less than the completeness decomposition temperature, so that there is produced composite silver nanoparticles which has a coating layer composed of organic material originated from said silver organic compound around a silver core formed by decomposition reduction. This production method is a solid-gas reaction. The particle diameter of said silver core is 1-100 nm, and therefore it is referred to composite silver nanoparticle as a popular name. For concreteness, when solid stearin acid silver 100 g is heated at 250□ for four hours in a flask under nitrogen flow, it is generated composite silver nanoparticles having the organic coating layer of stearin acid group around the silver core of particle diameter of 5 nm.
As a solid material of stearin acid silver is heated without a solvent in said production method, the generated composite silver nanoparticles are hard to disperse, and there is the defect that many composite silver nanoparticles become the big second particles because of their coupling in a dumpling state. Besides, it is found that the generation temperature is high with 250° C. and the metalization temperature of composite silver nanoparticles is extremely high with 220° C. The silver nanoparticles whose generation temperature is high comes to have the high silverizing temperature. Considering that the melting point of general Sn—Pb solder is 183° C. and the desired connection temperature is equal to or less than 200° C., the metalization temperature (silverizing temperature) is too high at 220° C., and it was difficult to employ as the substitute solder for low temperature. It can be thought that the reason why the metalization temperature is high is the huge particles with a dumpling state and the high decomposition temperature of stearin acid group. Besides, it is ensured from the inventor that the silver core is a simple atom ensemble or a polycrystal rather than a single crystal. In a case that said silver core is polycrystal or disorder ensemble, the electron scattering and thermal scattering occur on many grain boundary surfaces, and as a result, the electric conductivity and thermal conductivity are falling.
Next, Japanese Patent Laid-Open No. 2003-342605 bulletin is shown as patent document 2. Said patent document 2 is the invention that the present inventor also participated as one of the inventors. By dissolving or dispersing the metal organic compound in the organic solvent or water, it was successful to produce the composite silver nanoparticles coated by the organic material originated from the metal organic compound. This production method is a solid-liquid reaction. Besides, when these composite silver nanoparticles are observed in high resolution transmission electron microscope, the lattice image is confirmed in the silver core, so that it is found that the silver core is single crystal. This situation is confirmed. Based on the solid-liquid reaction method, it can be considered that the metal organic compound dissolves within the solvent as molecules, silver atoms are precipitated by reducing of said molecules and a single crystal appears by recombination between silver atoms. That is to say, it is thought that the single crystalline originates from intermolecular reaction. Because the silver core is a single crystal, there is an advantage that the electric conductivity and thermal conductivity are high. However, about the silverization temperature, it is written in [0076] that the composite silver nanoparticles of stearin acid group coating was heated at 250° C. for 10 minutes. In other words, it is considerably high temperature that the silverization temperature becomes 250° C.
This situation is a weak point of patent document 2. The reason why the silverization temperature is high is that the decomposition temperature of the organic acid group composing the coating layer is high because the reaction starts from the silver organic compound such as acetic acid silver, hexanoic acid silver and octane acid silver. It is necessary to improve the metalization temperature less than 200° C.
In there, WO00/076699 bulletin is shown as patent document 3. The present inventor is one of the inventors of this international publication bulletin. Plural inventions are disclosed in this publication bulletin, and especially in that, a production method treating a metal inorganic compound by means of surface active agent is shown for the first time, so that the way using a metal inorganic compound as the starting material was opened. That is to say, the above production method comprises the first step of forming the ultrafine particle precursor by colloidizing the metal inorganic compound in nonaqueous solvent by using of the surface active agent, and the second step of generating the composite metal nanoparticles having the surface active agent shell as the coating layer around the metal core by adding the reduction agent into said colloid solution and reducing said ultrafine particle precursor.
Because said method makes the nonaqueous solvent dissolve the metal inorganic compound, the generated composite metal nanoparticles have a feature hard to become the dumpling state since they disperse in the nonaqueous solvent. However, examples are composed of oleic acid copper, abietic acid silver, acetic acid silver, oleic acid nickel, diethyl hexane indium, copper acetate and stearin acid silver, and only organic metal compound is carried out. Besides, it was found that the metalization temperature of composite silver nanoparticles generated by stearin acid silver is high with 220° C. The further improvement making the metalization temperature less than 200° C. is necessary. In order to pursue the higher characteristic than Sn—Pb solder, more effort to make the metalization temperature less than 150° C. is required. Besides, since the judgement of single crystalline of polycrystalline of silver core is not done in the patent document 3, it is impossible to judge the good or bad of the electrical conductivity and thermal conductivity of composite metal nanoparticles.
Under the above-mentioned situation, WO01/070435 bulletin is shown as patent document 4. In this international publication bulletin, it is disclosed the composite metal nanoparticle obtained from metal salt comprising the metal core with particle diameter of 1 to 100 nm and the coating layer composed of organic compound including alcohol-related hydroxyl group larger than the carbon number 4 formed around said core. Moreover, the higher alcohol larger than the carbon number 6 is described as the organic compound including a functional group having the adsorption characteristic.
Furthermore, WO2005/075132 bulletin is shown as patent document 5. In this bulletin, it is disclosed the composite metal nanoparticle comprising the metal core at the center portion and the coating layer composed of organic material with the thermodesorption start temperature of the range of 140° C. to 190° C. around said core. As a production method, it is described the manufacturing of composite metal nanoparticle comprising the steps of coexisting the inorganic metal salt with the organic substance, forming the metal core by decomposition of the inorganic metal salt and forming the coating layer of organic material around it. In addition, it is also disclosed the composite metal nanoparticle formed the coating layer of organic material around the inorganic metal salt or the decomposed inorganic metal compound.
Japanese Patent Laid-Open No. 2007-95510 bulletin is shown as patent document 6. In claim 1 of patent document 6, it is also disclosed the electroconductivity paste comprising the organic solvent and the composite metal nanoparticles, which are composed of the metal cores formed by the metal component originated from the metal salt expressed with the chemical formula of (R-A)n-M and the organic coating layers originated from said metal salt. The R is hydrocarbon group with the carbon number of 4 to 9, A is COO, OSO3, SO3 or OPO3, and M is silver, gold or platinum. Therefore, the composite silver nanoparticles are included.
Japanese Patent Laid-Open No. 2004-107728 bulletin is disclosed as patent document 7. In claim 1 of patent document 7, it is described the composite metal nanoparticle having the organic coating layer composed of C, H and/or O as main component around the metal core with average particle diameter less than, and this organic coating layer is generated from the organic acid metal salt. This situation is described.    [Patent Document 1] U.S. Pat. No. 3,205,793 (Japanese Patent Laid-Open No. H10-183207)    [Patent Document 2] Japanese Patent Laid-Open No. 2003-342605    [Patent Document 3] WO00/076699    [Patent Document 4] WO01/070435    [Patent Document 5] WO2005/075132    [Patent Document 6] Japanese Patent Laid-Open No. 2007-95510    [Patent Document 7] Japanese Patent Laid-Open No. 2004-107728